The present invention relates to a method for realizing the delta-doping, with practical quality, in GaAs epitaxial layer grown on silicon substrate by metalorganic chemical vapor deposition (MOCVD).
Recently, heteroepitaxial technology concerned with the growth of GaAs on Si substrate, has been paid much attention. This is due to a few advantages of silicon substrate over GaAs substrate. Silicon substrate has sufficient mechanical hardness and high thermal conductivity. Large-area substrates with low defect density can be readily prepared with the present technology. In addition, silicon substrate is cheaper than GaAs substrate.
Specifically, the large-area availability enables the realization of tandem GaAs solar cells on Si. Moreover, mass production of cost-effective GaAs field effect transistors (FETs) on Si is also possible. Recently, a monolithic integrated circuit combining the Si memory devices and GaAs optoelectronic devices was suggested.
However, the problems arising from dissimilar properties between both materials are the major obstacles to prepare GaAs epitaxial layer on silicon substrate with acceptable quality. Many dislocations are inevitably involved in the GaAs epitaxial layer due to the relatively large lattice mismatch and thermal expansion coefficient difference of both materials. The dislocations are generated from GaAs/Si heterointerface and threaded to GaAs surface. Antiphase domain brought from the inherent problem when epitaxial growing of polar material (GaAs) on non-polar material (Si) in another problem.
A 2-step growth technique using a slightly tilted (100) silicon substrate has proven to be a successful way to grow high quality GaAs layer on Si. However, there still remains many dislocations with density of 10.sup.6 -10.sup.8 cm.sup.-2 near GaAs surface. For commercial GaAs devices on Si, much studies should be done.
Meanwhile, delta-doping concept which is a 2-dimensional doping concept has been recently emerged [E. F. Schubert, J. Vac. Technol. A8, 2980 (1990)]. This concept is clearly contrasted with the conventional way of doping (=3-dimensional doping). In this case, dopants are introduced into a reactor during growth interruption. After forming a dopant plane on GaAs surfaces, subsequent growth of undoped GaAs layer is followed. Due to the large electric field by ionized dopants, the conduction bands are significantly deformed and a V-shaped potential well is formed. Then very high density of 2-dimensional electrons can be confined in this unique potential well.
The delta-doping technique improves the characteristics of present devices. For example, when delta-doping layer is employed in an active region of a field effect transistor (FET), this delta-FET has superior characteristics as compared to a conventional FET. High source-drain saturation current (I.sub.dss), high transconductance (G.sub.m), and high reverse breakdown voltage are expected for the delta-FET.
The studies concerned with the delta-doping for GaAs epitaxial layer on GaAs substrate (homoepitaxy) have been mainly done using samples grown by molecular beam epitaxy (MBE). In that case, the growth temperatures for the delta-doped sample are typically in the range around 550.degree. C. Significant thermal diffusion of dopants in the delta-doped sheet during post growth has been observed when the growth temperature exceeds 550.degree. C. Thus significant degradation of delta-doping characteristics was observed.
If the same is true for MOCVD, since the nominal growth temperatures for MOCVD are in the range between 650.degree.-750.degree. C., the properties of MOCVD-grown delta-doped layer will be also deteriorated. However, due to a certain diffusion-limiting mechanism, we previously domonstrated the sucessful profiles of delta-doping for samples in nominal MOCVD-growth temperatures. The result for MOCVD-grown GaAs epitaxial layer on GaAs substrate was published elsewhere.
Developing this preliminary homoepitaxial research, the inventors discovered the technique of delta-doping in GaAs heteroepitazial layer grown on silicon substrate by MOCVD in the growing temperature of 700.degree.-750.degree. C.